The present invention relates generally to a method for recording and/or reproducing information into an optical recording medium capable of recording information in three-dimensional directions. More specifically, the invention relates to an information recording and/or reproducing method for continuously recording or reproducing marks or pits having recorded information, in accordance with a predetermined rule when the information is recorded or reproduced, an information recording apparatus using the optical recording medium, an information reproducing apparatus for reproducing the recorded information, and an optical recording medium used for recording and/or reproducing information.
As a typical medium capable of recording a large capacity of data, such as a high-density image, there is an optical recording disk. Although magneto-optical recording media and optical phase change media have been developed as optical recording disks, the need for increasing the capacity of information capable of being recorded into an optical recording disk is increasing.
In order to increase the capacity of the optical recording disk, a three-dimensional optical recording medium has been proposed. In this medium, the recording of information into an optical recording disk is allowed in, e.g., a depth direction as well as directions on a plane of the optical recording disk. In other words, a recording plane is multi-layered to three-dimensionally record information. While the optical recording disk has been given as an example of the three-dimensional optical recording medium, it should not be limited to a disk-shaped recording medium, but the three-dimensional optical recording medium may be a card-shaped recording medium or a stick-shaped recording medium as long as it records information in both of planar and depth directions.
The light transmittance of the above described magneto-optical recording medium and optical phase change medium have too low to realize a multilayer structure of tens to hundreds layers. However, there is some possibility that an optical recording medium using a photorefractive material, which will be described later, may realize a three-dimensional optical recording disk of tens to hundreds layers.
The first report on a three-dimensional optical recording medium is made by J. H. Strickler, et al., optical Letters, 16(22), 1789 (1991). This report discloses a three-dimensional mark type optical memory using an inorganic photorefractive material. The photorefractive material means a material having a refractive index changed by an electric field originated in a spatial distribution of electric charges which are generated by irradiating the electromagnetic waves. If such a material is applied to an optical memory, the intensity pattern of electromagnetic waves is recorded in the medium as the modulation in refractive index. The above-described first report discloses that an optical memory is irradiated with two beams to three-dimensionally write marks therein due to the photorefractive effect. With respect to a reproducing operation at this time, it has been reported that information was reproduced every one layer using a phase contrast microscope.
As the second report, Y. Kawata et al., Appl. Opt., 34, 4105 (1995) discloses that an optical memory is irradiated with one beam to three-dimensionally written marks therein due to the photorefractive effect. In the second report, a phase contrast microscope is also used for a reproducing operation.
As disclosed in the two reports, although a method using a phase contrast microscope for reproducing recorded information is suitable for a recording medium having a small cross section area, such as an inorganic crystal, it is not suitable for an optical recording medium having a large cross section area. That is, if this reading method is applied to a large-area disk-shaped optical recording medium, the area readable by a single shot is small, and a batch processing should be repeated. Therefore, there is a disadvantage in that a plurality of light irradiation operations for a reproducing operation must be carried out on the same plane, so that the readout rate is invariably slow.
A case where a recording and/or reproducing method, which has been carried out for a conventional optical recording disk, such as a compact disk, is applied to a three-dimensional optical recording disk will be considered. On the conventional optical recording disk, recording or reproducing light beams are spirally scanned in directions on the plane of the disk to record or reproduce information. That is, the optical recording disk is irradiated with the recording or reproducing light beams by moving a light source or the optical recording disk in radial directions while rotating the optical recording disk.
When this method is applied to the three-dimensional optical recording disk, the spiral recording or reproducing operation in the directions on the plane of the optical recording disk is carried out for one layer, and the recording or reproducing operation for one layer is repeated for a multi-layers, so that the three-dimensional recording operation can be carried out. However, in the method for causing the recording and reproducing operations for each layer to be repeated over the multi-layers, information is reproduced from all of the layers of the disk, so that the readout rate is slow. Because, after the reproducing operation for the first layer, it takes a long period to start the reproducing operation for the next layer.
That is, in order to start the reproducing operation for the second layer after the reproducing operation for the first layer is completed, a process for detecting the position of the mark in the second layer of the optical recording disk in the depth direction is required. Specifically, the position for focused beams in the disk is changed by vertically scanning a lens for condensing readout laser or vertically moving the disk, and simultaneously, the intensity of the reproducing laser is measured to detect a mark, at which the modulation in intensity is largest or a point, at which the modulation in differential of the intensity of the reproducing light is largest. It is considered that the reason why it takes a lot of time for a reproducing operation is that the conventional reproducing method requires a process for detecting a focal point in the depth direction.
Thus, in this technical field, it is desired to develop a three-dimensional optical recording medium capable of three-dimensionally recording information for high-capacity recording into the optical recording disk, and an information recording and/or reproducing method, an information recording apparatus and an information reproducing apparatus, which can rapidly carry out recording and reproducing rates using the optical recording medium. However, this has not been realized. If such an information recording and/or reproducing method is not provided, even if it is possible to record high-capacity information in the three-dimensional optical recording medium in any ways, it is not possible to carry out a rapid recording or reproducing rate which is fit for practical use. In addition, even if a high functional recording medium is developed, it can not been effectively utilized.